Total U.S. greenhouse gas emissions rose in 1997 to 1,813.6 million metric tons of carbon equivalents (MMTCE)
3
(11.1 percent above 1990 baseline levels). The single year increase in emissions from 1996 to 1997 was 1.3 percent (23.1 MMTCE), down from the previous year's increase of 3.3 percent. Figure ES-1 through Figure ES-3 illustrate the overall trends in total U.S. emissions by gas, annual changes, and absolute change since 1990. Table ES-1 provides a detailed summary of U.S. greenhouse gas emissions and sinks for 1990 through 1997.

+
Does not exceed 0.05 MMTCE
a
Sinks are only included in net emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities exclude non-forest soils, and are based partially upon projections of forest carbon stocks.
b
Emissions from International Bunker Fuels are not included in totals.
Note: Totals may not sum due to independent rounding.

Figure ES-4: 1997 Greenhouse Gas Emissions by Gas
Figure ES-4 illustrates the relative contribution of the direct greenhouse gases to total U.S. emissions in 1997. The primary greenhouse gas emitted by human activities was CO
2
. The largest source of CO
2
and of overall greenhouse gas emissions in the United States was fossil fuel combustion. Methane emissions resulted primarily from decomposition of wastes in landfills, manure and enteric fermentation associated with domestic livestock, natural gas systems, and coal mining. Emissions of N
2
O were dominated by agricultural soil management and mobile source fossil fuel combustion. The substitution of ozone depleting substances and emissions of HFC-23 during the production of HCFC-22 were the primary contributors to aggregate HFC emissions. PFC emissions came mainly from primary aluminum production, while electrical transmission and distribution systems emitted the majority of SF
6
.
As the largest source of U.S. GHG emissions, CO
2
from fossil fuel combustion accounted for 81 percent of emissions in 1997 when each gas is weighted by its
Global Warming Potential
. Emissions from this source grew by 11 percent (138.8 MMTCE) from 1990 to 1997 and were responsible for over three-quarters of the increase in national emissions during this period. The annual increase in CO
2
emissions from this source was 1.3 percent in 1997, down from the previous year when emissions increased by 3.6 percent.
The dramatic increase in fossil fuel combustion-related CO
2
emissions in 1996 was primarily a function of two factors: 1) fuel switching by electric utilities from natural gas to more carbon intensive coal as gas prices rose sharply due to weather conditions, which drove up residential consumption of natural gas for heating; and 2) higher petroleum consumption for transportation. In 1997, by comparison, electric utility natural gas consumption rose to regain much of the previous year’s decline as the supply available rose due to lower residential consumption. Despite this increase in natural gas consumption by utilities and relatively stagnant U.S. electricity consumption, coal consumption rose in 1997 to offset the temporary shut-down of several nuclear power plants. Petroleum consumption for transportation activities in 1997 also grew by less than one percent, compared to over three percent the previous year (see Table ES-2). The annual increase in CO
2
emissions from petroleum in 1997 is based on motor gasoline sales data from the U.S. Energy Information Administration; it is expected to be revised upward with the publication of future energy statistics.

Overall, from 1990 to 1997, total emissions of CO
2
, CH
4
, and N
2
O increased by 143.5 (11 percent), 9.7 (6 percent), and 13.4 MMTCE (14 percent), respectively. During the same period, weighted emissions of HFCs, PFCs, and SF
6
rose by 14.9 MMTCE (67 percent). Despite being emitted in smaller quantities relative to the other principle greenhouse gases, emissions of HFCs, PFCs, and SF
6
are significant because of their extremely high Global Warming Potentials and, in the cases of PFCs and SF
6
, long atmospheric lifetimes. Conversely, U.S. greenhouse gas emissions were partly offset by carbon sequestration in forests, which was estimated to be 11 percent of total emissions in 1997.
Other significant trends in emissions from additional source categories over the eight year period from 1990 through 1997 included the following:

º

Aggregate HFC and PFC emissions resulting from the substitution of ozone depleting substances (e.g., CFCs) increased dramatically (by 14.4 MMTCE). This increase was partly offset, however, by reductions in PFC emissions from aluminum production (41 percent) and HFC emissions from HCFC-22 production (14 percent), both as a result of voluntary industry emission reduction efforts and, in the former case, from falling domestic aluminum production.

An additional domestic adipic acid plant installed emission control systems in 1997; this was estimated to have resulted in a 1.4 MMTCE (27 percent) decline in emissions from 1996 to 1997 despite an increase in production.

There are several ways to assess a nation’s greenhouse gas emitting intensity. These measures of intensity could be based on aggregate energy consumption because energy-related activities are the largest sources of emissions, on fossil fuel consumption only because almost all energy-related emissions involve the combustion of fossil fuels, on electricity consumption because electric utilities were the largest sources of U.S. greenhouse gas emissions in 1997, on total gross domestic product as a measure of national economic activity, or on a per capita basis. Depending upon which of these measures was used, the United States could appear to have reduced or increased its national greenhouse gas intensity. Table ES-3 provides data on various statistics related to U.S. greenhouse gas emissions normalized to 1990 as a baseline year. Greenhouse gas emissions in the U.S. have grown at an average annual rate of 1.5 percent since 1990. This rate is slightly slower than that for total energy or fossil fuel consumption – thereby indicating an improved or lower greenhouse gas emitting intensity – and much slower than that for either electricity consumption or overall gross domestic product. Emissions, however, are growing faster than national population, thereby indicating a worsening or higher greenhouse gas emitting intensity on a per capita basis (see Figure ES-5). Overall, atmospheric CO
2
concentrations – a function of many complex anthropogenic and natural processes – are increasing at 0.4 percent per year.

Figure ES-5
U.S. Greenhouse Gas Emissions
Per Capita and Per Dollar
of Gross Domestic Product

Box ES-2: Greenhouse Gas Emissions from Transportation Activities

Motor vehicle usage is increasing all over the world, including in the United States. Since the 1970s, the number of highway vehicles registered in the United States has increased faster than the overall population, according to the Federal Highway Administration. Likewise, the number of miles driven—up 18 percent from 1990 to 1997—and gallons of gasoline consumed each year in the United States have increased relatively steadily since the 1980s, according to the Energy Information Administration. These increases in motor vehicle usage are the result of a confluence of factors including population growth, economic growth, increasing urban sprawl, and low fuel prices.
One of the unintended consequences of these changes is a slowing of progress toward cleaner air in both urban and rural parts of the country. Passenger cars, trucks, motorcycles, and buses emit significant quantities of air pollutants with local, regional, and global effects. Motor vehicles are major sources of carbon monoxide (CO), carbon dioxide (CO
2
), methane (CH
4
), nonmethane volatile organic compounds (NMVOCs), nitrogen oxides (NOx), nitrous oxide (N
2
O), and hydrofluorocarbons (HFCs). Motor vehicles are also important contributors to many serious air pollution problems, including ground-level ozone or smog, acid rain, fine particulate matter, and global warming. Within the United States and abroad, government agencies have taken strong actions to reduce these emissions. Since the 1970s, the EPA has reduced lead in gasoline, developed strict emission standards for new passenger cars and trucks, directed states to enact comprehensive motor vehicle emission control programs, required inspection and maintenance programs, and more recently, introduced the use of reformulated gasoline to mitigate the air pollution impacts from motor vehicles. New vehicles are now equipped with advanced emissions controls, which are designed to reduce emissions of nitrogen oxides, hydrocarbons, and carbon monoxide.
This report reflects new data on the role that automotive catalytic converters play in emissions of N
2
O, a powerful greenhouse gas. The EPA’s Office of Mobile Sources has conducted a series of tests in order to measure the magnitude of N2O emissions from gasoline-fueled passenger cars and light-duty trucks equipped with catalytic converters. Results show that N
2
O emissions are lower than the IPCC default factors, and the United States has shared this data with the IPCC. In this report, new emission factors developed from these measurements and from previously published literature were used to calculate emissions from mobile sources in the United States (see Annex C).
Table ES-4 summarizes greenhouse gas emissions from all transportation-related activities. Overall, transportation activities—excluding international bunker fuels—accounted for an almost constant 26 percent of total U.S. greenhouse gas emissions from 1990 to 1997. These emissions were primarily CO
2
from fuel combustion, which increased by 10 percent from 1990 to 1997. However, because of larger increases in N2O and HFC emissions during this period, overall emissions from transportation activities actually increased by 12 percent.

Table ES-4: Transportation Related Greenhouse Gas Emissions (MMTCE)

Gas — Vehicle Type

1990

1991

1992

1993

1994

1995

1996

1997

CO
2

405.0

396.7

402.4

406.8

422.1

430.7

445.3

446.5

Passenger Cars
a

169.3

167.8

172.0

173.5

172.5

175.6

160.8

162.6

Light-Duty Trucks
a

77.5

77.2

77.2

80.5

87.2

89.2

109.9

111.1

Other Trucks

57.3

55.1

56.7

59.9

62.7

64.2

68.3

69.5

Buses

2.7

2.9

2.9

3.1

3.3

3.5

3.0

3.0

Aircraft

50.5

48.4

47.4

47.6

49.6

48.3

50.5

50.1

Boats and Vessels

16.4

15.9

16.4

11.7

13.9

16.8

18.5

15.4

Locomotives

7.5

6.9

7.4

6.8

8.0

8.1

8.8

9.0

Other
b

23.8

22.5

22.4

23.8

24.9

24.9

25.5

25.8

International Bunker Fuels
c

27.1

27.8

29.0

29.9

27.4

25.4

25.4

26.6

CH
4

1.4

1.4

1.4

1.4

1.4

1.4

1.4

1.4

Passenger Cars

0.8

0.7

0.7

0.7

0.7

0.7

0.6

0.6

Light-Duty Trucks

0.4

0.4

0.4

0.4

0.4

0.4

0.5

0.5

Other Trucks and Buses

0.1

0.1

0.1

0.2

0.2

0.2

0.2

0.2

Aircraft

+

+

+

+

+

+

+

+

Boats and Vessels

+

+

+

+

+

+

+

+

Locomotives

+

+

+

+

+

+

+

+

Other
d

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

International Bunker Fuels
c

+

+

+

+

+

+

+

+

N
2
O

13.6

14.2

15.2

15.9

16.7

17.0

17.4

17.5

Passenger Cars

8.7

9.1

9.7

10.1

10.0

10.1

8.9

9.1

Light-Duty Trucks

3.4

3.7

3.9

4.2

5.1

5.2

6.8

6.8

Other Trucks and buses

0.7

0.7

0.7

0.7

0.8

0.8

0.9

0.9

Aircraft
d

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

Boats and Vessels

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

Locomotives

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

Other
d

0.2

0.2

0.2

0.2

0.2

0.2

0.2

0.2

HFCs

+

+

0.2

0.7

1.3

2.5

3.6

4.5

Mobile Air Conditioners
e

+

+

0.2

0.7

1.3

2.5

3.6

4.5

Total

420.0

412.3

419.1

424.8

441.5

451.6

467.7

469.9

+ Does not exceed 0.05 MMTCE.
Note: Totals may not sum due to independent rounding.
a
In 1996, the U.S. Federal Highway Administration modified the definition of light-duty trucks to include minivans and sport utility vehicles. Previously, these vehicles were included under the passenger cars category. Hence the sharp drop in CO2 emissions for passenger cars from 1995 to 1996 was observed. This gap, however, was offset by an equivalent rise in CO
2
emissions from light-duty trucks.
b
“Other” CO
2
emissions include motorcycles, construction equipment, agricultural machinery, pipelines, and lubricants.
c
Emissions from International Bunker Fuels are not included in totals.
d
“Other” CH
4
and N
2
O emissions include motorcycles, construction equipment, agricultural machinery, gasoline-powered recreational, industrial, lawn and garden, light commercial, logging, airport service, other equipment; and diesel-powered recreational, industrial, lawn and garden, light construction, airport service.
e
Includes primarily HFC-134a

Box ES-3: Electric Utility Related Greenhouse Gas Emissions

Like transportation, activities related to the generation, transmission, and distribution of electricity in the United States result in greenhouse gas emissions. Table ES-5 presents greenhouse gas emissions from electric utility-related activities. Aggregate emissions from electric utilities of all greenhouse gases increased by 11.8 percent from 1990 to 1997, and accounted for just under 30 percent of total U.S. greenhouse emissions during the same period. The majority of these emissions resulted from the combustion of coal in boilers to produce steam that is passed through a turbine to generate electricity. Overall, the generation of electricity results in a larger portion of total U.S. greenhouse gas emissions than any other activity.

Table ES-5: Electric Utility Related Greenhouse Gas Emissions (MMTCE)

Gas — Fuel Type or Source

1990

1991

1992

1993

1994

1995

1996

1997

CO
2

476.8

473.4

472.5

490.7

494.8

494.1

513.2

532.3

Coal

409.0

407.2

411.8

428.7

430.2

433.0

457.5

470.9

Natural Gas

41.2

41.1

40.7

39.5

44.0

47.2

40.3

43.8

Petroleum

26.6

25.1

19.9

22.5

20.6

14.0

15.4

17.6

Geothermal

0.1

0.1

0.1

0.1

+

+

+

+

CH
4

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

Stationary Sources (Utilities)

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

N
2
O

2.0

2.0

2.0

2.1

2.1

2.1

2.2

2.3

Stationary Sources (Utilities)

2.0

2.0

2.0

2.1

2.1

2.1

2.2

2.3

SF
6

5.6

5.9

6.2

6.4

6.7

7.0

7.0

7.0

Electrical Transmission and Distribution

5.6

5.9

6.2

6.4

6.7

7.0

7.0

7.0

Total

484.6

481.4

480.8

499.3

503.7

503.3

522.5

541.7

+ Does not exceed 0.05 MMTCE.
Note: Totals may not sum due to independent rounding.
The following sections describe the concept of Global Warming Potentials (GWPs), present the anthropogenic sources and sinks of greenhouse gas emissions in the United States, briefly discuss emission pathways, summarize the emission estimates, and explain the relative importance of emissions from each source category.

3. Estimates are presented in units of millions of metric tons of carbon equivalents (MMTCE), which weights each gas by its GWP value, or
Global Warming Potential
.

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